These indexes measure things like "enforceability of contracts", "existence of black market" 'Having to pay bribes to low level staff to get things done" "Existence of transparent procurement processes in government departments" and things like that. They do not measure political freedom, and while it is true that countries with low political freedom are generally very corrupt, Singapore is probably the exception rather than the norm.
One would generally agree that in Singapore, as with Australia, there is strong enforeceability of contracts, no existence of black markets, there is no need to pay bribes to low level staff to get things done, and there exists transparent procurement processes in government departments.
Corruption in developed countries generally takes place in such clever and subtle ways, that it is impossible to prove. For example, if you have access to a perfect flow of information, why do you need to take a cent from anyone? And most politicians and high ranking civil servants know things before anyone else.
The Qantas Group (incl Jetstar) hub in Singapore has brought a lot of benefits to Singapore's economy, and has helped grow traffic through SIN, as SIA's pax numbers have only grown 1% per annum for the past decade. Without the QF Group hub here, if we only depended on SQ, overall pax traffic would have stagnated. There are also routes where Jetstar has more frequencies than SQ (4 to Ho Chi Minh instead of 2 for SQ) and to Jakarta - TR has 2 and 3K/VF have up to 4 on some days too.
The most ridiculous thing though is QF still has not started to coordinate schedules with 3K. You can't book a London-Bali ticket on the QF website, neither can you book London-Penang, London-Phuket and so on.